This invention is related generally to achieving better adhesion between a protective coating and a target substrate, and in particular to creating a tribologically suitable surface in engine cylinder bores by mechanical roughening the bores in such a way that internal stresses in the bores and resultant cracking in a subsequently-applied thermal sprayed coating are reduced.
The cylinder walls and cylinder liners of an internal combustion engine (ICE) are manufactured to exacting standards with close tolerances as a way to promote efficient engine operation. While additional increases in efficiency may also be realized through hotter, more complete combustion processes, the increased thermal load imparted to the walls and liners (also referred to collectively or individually as bores) of the engine block provides additional structural and related durability challenges to lightweight, efficient engine designs.
Thermal spray techniques have been shown to be an effective way to deposit protective coatings—such as thermal barrier coatings (TBCs), wear resistant coatings, anti-corrosion coatings or the like—onto the bores. Adhesion of the protective coating to a substrate is a very important metric for determining the suitability of the coating for a particular application (such as for the harsh environments produced within the combustion chamber of an internal combustion engine cylinder bore). Traditional approaches for enhanced coating adhesion to the cylinder bore substrate involve various surface activation pretreatment steps, including approaches such as grit blasting with ceramic particles and high-pressure water jet blasting. Grit blasting, while effective, leaves behind particle residue that can contaminate subsequent coating application steps unless costly and time-consuming cleansing steps are also employed. Water jet blasting, while less likely to leave behind undesirable byproducts, uses large quantities of water, or requires a complex water treatment system for water recycling. Moreover, the presence of contaminants or byproducts within the water once the roughening operation is complete make it undesirable to dispose of the spent water back into a local aquatic environment. Furthermore, the high-pressure water jet blasting approach has high capital costs.
A more recent development promises to achieve protective coating adhesion results similar to grit blasting and water jet blasting, but without the drawbacks. Mechanical roughening/locking involves carving geometric shapes out the bore wall with cutting machinery through one or more of chipping, pressing, sliding, rolling and related steps. Such roughening changes the topography on the substrate surface to promote an interlocking fit between the coating and the substrate. In one such form, trapezoidal or dovetail-shaped undercuts are formed in the roughened bore surface to promote this interlocking fit. An example of such an approach may be found in U.S. Published Application 2012/0317790 (hereinafter the '790 Publication) filed by Flores, Baumgartner and Rach and entitled TOOL AND METHOD FOR MECHANICAL ROUGHENING the entirety of which is hereby incorporated by reference.
A significant problem with mechanical roughening is that large amounts of internal stresses are generated in the substrate, especially in the region nearest to the surface. This in turn may result in a high tensile stresses and concomitant shear loads between the substrate and the subsequently-applied coating in the bore's axial and tangential directions, where such stresses and related loads cause cracking that is especially detrimental to the performance and durability of thermal spray coatings. As such, the present inventors believe that there is a need for an approach to cylinder bore pretreatment to permit a higher integrity bond between the mechanically-roughened bore and a protective coating placed on the bore through the reduction of residual stresses in the roughened bore substrate.